1. Field of the Invention
The present invention relates in general to methods and apparatus for testing material samples which are initially fluid but which may change to a solid during testing. More specifically, the present invention relates to method and apparatus for determining the gel strength and compressive strength of a cement slurry sample.
2. Description of the Prior Art
Cement is utilized at different times during the drilling, completion, and repair of oil and gas wells to bond the well casing to the well bore. This technique is utilized to seal off any flow of fluids or gas along the length of the casing. The cement utilized is formulated specifically for the temperature and pressure conditions found in the well being cemented. Because of the wide range of conditions present in typical oil wells there are a wide range of diverse cement formulations utilized in this cementing procedure. In order to ensure proper cementing within the well the cement utilized is typically tested in a laboratory utilizing the specific conditions believed to be present in a particular well. Multiple measurement systems are described, for example, in U.S. Pat. No. 4,259,868 issued to Rao, et al.; U.S. Pat. No. 4,567,765 to Rao, et al.; U.S. Pat. No. 5,412,990 to D'Angelo, et al.; and U.S. Pat. No. 5,992,223 to Sabins, et al.
A primary requirement in such operations is that the cement slurry remain fluid for a period long enough to permit the slurry to be pumped to desired locations within the well. A high temperature-high pressure consistometer is often utilized to measure the thickness or consistency of the cement during the pumping operation. Once in place the cement must develop adequate gel strength to prevent fluid or gas movement before the cement develops compressive strength. Eventually, it is necessary to know that adequate compressive strength has developed before beginning certain other operations within an oil well. An ultrasonic cement analyzer may be utilized to measure the gel strength and the compressive strength of the cement as it cures. Previous versions of devices utilized to obtain these measurements typically utilized one or two acoustic transducers to measure the sound velocity within the cement to obtain the compressive strength or the amplitude of the signal through the cement in order to obtain the gel strength.
These prior art devices typically utilized relatively large samples of cement contained within a pressure vessel. After the cement has set and the test is complete the pressure vessel would then be disconnected from the high pressure fluid source and electronics to allow its removal from the housing which contains the electronics, pressure source and heat source. These pressure vessels, typically weighing 20 lbs. or more, would then be taken to a large bench vice to allow removal of both the top and bottom plugs from the pressure vessel. The cement sample may then be driven from the pressure vessel. Once the cement has been removed grease is typically applied to all interior surfaces of the pressure vessel before filling the pressure vessel with a subsequent cement sample. This test equipment does not lend itself to affordability or portability in view of the massive pressure vessels required. Additionally, this test equipment is quite expensive.